The present invention relates to wavefront sensors for measuring phase distortions in a wavefront of light.
The resolution of ground based optical imaging systems is limited by random wavefront tilts and phase changes produced by atmospheric turbulence. The resolution of such optical systems is usually limited to about one or two arc seconds by the atmosphere and may be considerably improved if the atmospheric distortion can be measured and corrected in real-time prior to recording of the image on tape or film.
A system for real-time optical wavefront compensation employing discrete components to perform the wavefront sensing and wavefront compensation functions is disclosed in U.S. Pat. No. 3,923,400, issued to John W. Hardy for A Real-Time Wavefront Correction System, which is incorporated by reference herein. In the approach disclosed in this patent, a wavefront being examined is directed onto an AC lateral shearing interferometer which measures in real time the relative phase differences between different portions of the wavefront. The constructional details of a suitable lateral AC shearing interferometer are disclosed in Wyant U.S. Pat. No. 3,829,219 for Shearing Interferometer incorporated by reference herein. The measured phase differences in the form of a first set of electrical signals are then directed to a data processor which generates a second set of electrical signals, the amplitudes of which are proportional to the required phase corrections at the different areas of the wavefront. The second set of signals may be directed to a separate phase corrector element upon which the wavefront is incident to eliminate the relative phase differences of the wavefront. One type of separate phase corrector element disclosed by this patent is an active mirror having an array of piezoelectric elements incorporated therein which function to selectively deform the mirror surface to eliminate phase distortions in the wavefront. The constructional details of an active piezoelectric mirror of this nature are disclosed in U.S. Pat. No. 3,904,274 for Monolithic Piezoelectric Wavefront Phase Modulator incorporated by reference herein.
At other times the shearing interferometer may be employed in the absense of such compensation to measure phase distortion of the wavefront per se, or distortions of an optical system through which the light is passed.
The Wyant interferometer of the '219 patent, is often employed with a shear equal to one sub-aperture. While the results of the measurements are quite accurate, the dynamic range of the wavefront sensor is only plus or minus one half of a wavelength per sub-aperture. If the shear is reduced for example to one quarter wavelength per sub-aperture, the dynamic range is considerably expanded to typically plus or minus two wavelengths per sub-aperture. However, the resulting set of measurements have far less accuracy, due to the reduced shear, and less spatial resolution. Since a typical phase distortion or tilt in the slope of the wavefront passing through the atmosphere can be plus or minus two wavelengths per sub-aperture or more, it is thus an object of the present invention to provide a method of obtaining with a single interferometer, both a relatively large dynamic range in the measurements, of at least plus or minus two wavelengths per sub-aperture, and yet retain the accuracy of the large shear interferometer having the restricted dynamic range of plus or minus one half of a wavelength per sub-aperture.